52 THE MRCHANICAL FUNCTIONS. 



As the living functions imply the mechanical 

 action and re-action of parts which cohere in some 

 definite order of arrangement, so as to preserve 

 that determinate form to which they constantly 

 tend to return on being displaced, it is impossible 

 to conceive that a mere fluid can exercise these 

 functions ; because the particles of a fluid, being 

 equally moveable in every direction, have no deter- 

 minate relative situations, and possess no character 

 of permanence. All organic and living structures, 

 therefore, must be composed of solid as well as 

 fluid parts ; although the proportion between these 

 is, in different cases, almost infinitely varied. A 

 dormant vitality may, indeed, exist in a system of 

 organs which have been brought into a perfectly 

 dry state ; as is proved by the examples of vege- 

 table seeds, and also of many species of animal- 

 cules, and even of some of the more highly organized 

 Annelida, or worms, which may be kept in a dry 

 state for an indefinite length of time, and, when 

 moistened with water, resume their activity, as if 

 restored to life. The germination of seeds imder 

 these circumstances is matter of common observa- 

 tion; but the revivification of animalcules is a more 

 curious phenomenon, for it takes place more ra- 

 pidly, and is more striking in its results. The 

 Rotifer vulgaris (Ehr.) or wheel -animalcule,* 

 (Fig. 1.) which was first observed by Lewenhoeck, 

 and was afterwards rendered celebrated by the ex- 



* Vorticella rotatoria of Muller, and Furcularia rediviva of 

 Lamarck. This animalcule was first described by Eichhorn, in 

 1767, under the name of Wasserbar, or water bear. Corti, in 

 1774, first observed the phenomenon of its reviviscence after desic- 

 cation. Spallanzani's researches on the same subject were made in 



